The present invention relates to a process for the removal of volatile constituents from polymers. More particularly, the present invention relates to a multiple-stage flash devolatilization process for removing and recovering volatiles from polymers, especially mass processable polymers, which enables optimally low pressures to be employed without the necessity for refrigeration to condense the volatile constituents recovered thereby.
In the preparation of polymeric materials, particularly the mass processable polymers, considerable amounts of unreacted monomer and low molecular weight polymers, such as dimer and trimers, remain admixed or entrained in the polymer product. Due to the considerable difficulty in polymerizing a polymerization mixture to absolute completion, complete polymerization of all the available monomer has been found to be impractical on a commercial basis. Moreover, polymerization of the final portion of available monomer has been found to have a deleterious effect on polymer product quality, resulting in the production of undesirable low molecular weight polymer. Additionally, it is also common practice to employ various inert solvents and other agents during the polymerization process in order to achieve certain results in the operating conditions and in the characteristics of the polymer product. The presence of such solvents as contaminants in the final polymer product has an adverse effect on the polymer properties, tending to cause crazing in the polymer, decreased heat distortion temperature, and increased odor. Additionally, the volatile or fugacious constituents of the polymeric material are susceptible to leaching, precluding use of the polymer product in conjunction with foods, for example, as for packaging containers and the like. Accordingly, it is common practice in the art to subject the polymeric materials to a devolatilization step in which a stream of hot polymer product is exposed to reduced pressure in order to vaporize and remove the volatile components from the polymer product.
In order to produce the highest quality product, it is desirable to reduce the residual volatile level of the finished polymer to as low a level as possible. For many commercial applications, such as, food packaging, Federal regulations furthermore require that the finished polymer product be substantially free of volatile constituents. Maximum volatiles content can be as low as a few parts per million by weight volatiles. To this end, the prior art has developed many devolatilization processes for reducing the volatiles content of polymeric materials, and particularly for reducing the volatiles content of mass processable polymers. Representative examples of these processes include the devolatilization processes of U.S. Pat. Nos. 2,941,985; 3,311,076; and 3,668,161. The first and last, in particular, teach that devolatilization may be enhanced through the use of a multiple-stage flash devolatilization process in which the polymeric material is subjected to successively lower pressures.
U.S. Pat. No. 3,073,812 teaches a process wherein ethylene and propylene polymers are precipitated from solution in a three-stage flash cooling process. Such a process, however, is unsuitable for use in the devolatilization of mass processable polymers, such as styrene polymers and copolymers, since the low temperatures used therein preclude reduction of the volatiles content of the polymer to desirable levels. Similarly, U.S. Pat. No. 3,280,091 teaches a process for recovering unreacted propylene from a polypropylene polymer solution by a two-stage flash devolatilization combined with a subsequent monomer stripping step. The process of this patent is likewise unsuitable for use in the devolatilization of mass processable polymers since a solvent stripping step subsequent to flashing re-contaminates the polymeric material with a volatile solvent contaminant.
In order to reduce the volatiles content of a polymeric material to successively lower levels, it is necessary to expose the polymeric material to increasingly lower vacuum. As the pressure in the flash devolatilization zones is decreased to the ultimately low pressures necessary to substantially completely remove the volatiles from the polymeric material, refrigeration of the volatiles condenser becomes necessary. Actually you cannot operate in any practical way without condensation for your vacuum producing equipment would have to be large enough to pass all vapor at the reduced pressure, a mind-bogging volume. Heretofore, therefore, the prior art has required the use of a refrigerated condenser on the second stage low pressure flash devolatilization zone in a multiple-stage devolatilization process. In U.S. Pat. No. 2,941,985, for example, the majority of the volatiles are removed in a first relatively higher pressure flash devolatilization zone, and the residual volatiles are removed in a second optimally low pressure devolatilization zone. However, the requirement for expensive refrigeration to condense the volatiles in the above processes renders their use undesirable on a commercial basis. Accordingly, there exists a need for a process for the devolatilization of polymers, particularly mass processable polymers, which can achieve substantially complete removal of the volatile constituents from the polymeric material without the necessity for refrigeration.